A Mermaid's Tears (Fred Weasley)
But a mermaid has no tears, and therefore she suffers so much more.
Marceline has been shunned her whole life. She can't live with her mother in the sea, and her dad was killed when Marceline was a young girl. She lives with her grandparents, but they don't like the idea of a half-mermaid being their granddaughter. Hogwarts is a bittersweet option for Marceline. While she'll be able to get away from a home where she feels unwelcome, she doesn't know how people will treat her at school if she's a half-mermaid.
But who says they have to find out?